Television brightness and contrast control circuit



April 2o, 1965 TELEVISION BRIGHTNESS AND CONTRAST CONTROL CIRCUITOriginal Filed March 27, 1959 R. w. AHRoNs ETAL 3,179,743

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nited States Patent O 3,179,743 TELEVISION ERIGHTNESS ANI) CONTRASTCONTROL CIRCUIT Richard W. Ahrons, Somerville, and Leslie L. Burns, Jr.,Princeton, NJ., assignors to Radio Corporation of America, a corporationof Delaware Original application Mar. 27, 1959, Ser. No. 802,336, nowPatent No. 3,072,741, dated `Ian. 8, 1963. Divided and this applicationMay 15, 1962, Ser. No. 194,827

7 Claims. (Cl. 178-5.4)

This invention relates to an automatic system for controlling thebrightness and contrast of the image reproduced by a television imagereproducing device.

This application is a division of an application Serial No. 802,386,filed March 27, 1959, now Patent No. 3,072,741, issued January 8, 1963for Richard W. Ahrons and Leslie L. Burns, Jr., entitled TelevisionBrightness and Contrast Control Circui and assigned to the same assigneeas this application.

In order to make the description of the invention that follows clear, itshould be kept in mind that by contrast control is meant the control,generally on a television receiver or monitor, that principally controlsthe amount of peak-to-peak video signal applied to the image reproducingdevice. Similarly, the brightness control operates to vary the amount ofdirect our-rent (DC.) voltage in the signal that is applied to the imagereproducing device. Because the low level or dark portions of thepicture are particularly sensitive to the absolute level at which theyoccur, the brightness control is often referred to as a backgroundcontrol. j

Brightness and contrast controls are generally included on a televisionreceiver, for example, primarily to enable the viewer to correct forprogram and station differences as well as aging of the receivercomponents. Program differences, created for example by very poor grayscale rendition (resulting when old movies are being viewed), may becorrected to some extent by variation of the brightness and contrastsettings of a television receiver. Since some television transmittingstations operate with more set up, some adjustment of the brightnesscontrol on the usual television receiver is often required to obtain asuitable background of the reproduced image. Also, some stations tend tomodulate at a higher level than others which requires a suitableadjustment of the contrast control.

Although brightness and contrast controls are useful, the average viewerdoes not understand these controls well enough to be able to obtain anoptimum picture except by a process of trial and error. Both thecontrast and brightness controls interact with each other to the extentthat each alfects the operation of the other. This interaction of thecontrols is considered desirable by some engineers but on the other handmay so confuse the viewer that he may not consistently obtain optimumperformance from the television receiver.

It has been found that on the average, if the viewer is not allowed toadjust any of the controls, a more satisfactory picture can be obtained.The reason for this statement may be illustrated by a simple example.Suppose the viewer turns up the contrast to compensate for lowmodulation. Next, suppose he later changes to a station that modulatesfully and observes that the blacks are too dark and then turns up thebrightness control. Under these conditions, if the scene next changes toone with high overall brightness, the receiver high voltage power supplycould immediately overload and cause defocussing, blooming or, if thetelevision receiver is a color set, turn the entire picture to green.

Accordingly, it is an object of this invention to provide a j 53,179,743Patented Apr. 20, 1965 novel and improved cathode ray tube automaticcontrast and brightness control system.

Current practice in color television receivers and color monitors usingshadow-mask color kinescopes requires operating the kinescope at or nearits maximum ratings for high voltage power input. In order to providegood regulation, the high voltage supply should be capable of providingconsiderably more than the safe power input to the kinescope. A monitoror receiver operating this way can be overloaded and possibly damagedbythe simple condition of applying excessive level of video signal tothe kinescope.

In a typical high voltage supply for a color kinescope,` high voltageyback pulses, derived from the horizontal deflection output transformer,are applied to the input electrode of a rectifier. The rectifierdelivers a charging current to a capacitor (which may be part of theukinescope) connected between the output electrode of the rectifier and apoint of reference potential in response to the rectification of theilyback pulses. The D.C. Voltage developed across the charging capacitoris applied to the kinescope ultor electrode. Since the effective loadingon the ultor supply will necessarily vary with picture content, i.e.with beam current, D.C. regulation is generally required. Suitablevoltage regulation is achieved by shunting the space discharge path of aregulator tube across the rectifier output circuit. However, suchregulator requires an additional tube that is usually relativelyexpensive and does nothing toward simplifying the brightness andcontrast controls in the receiver.

Hence another object of the invention is to provide a novel and improvedcircuit for maintaining constant the average cathode ray beam current ina cathode ray tube.

A further object of t-he invention is to provide a novel and improvedcircuit for automatically controlling the brightness and contrast of thepicture produced by a television reproducing system.

An additional object of the present invention is to provide an improved.system that .prevents spot blooming in a color television receiver. t

Still another object of this invention is to provide a novel system thatprevents ultor power supply overload in a color television receiverwhereby the need for a shunt regulator tube in `the high voltage powersupply is eliminated.

An additional object of this invention is to provide a novel automaticbrightness and contrast control circuit for a television signalreproducing device which circuit is insensitive to the aging of thetelevision reproducing device and the control circuit itself.`

In accordance'with one form of the invention, two feedback loops areemployed in a television receiver; one to maintain the average currentin lthe kinescope (image reproducing device) constant; the other toprevent the white peaks of the video signal driving the kinescope fromexceeding the level at which spot blooming occurs.

As noted above, it is normally desirable to operate a` A more detaileddescription follows, in conjunctionl with the accompanying drawings, inwhich like reference numerals refer to like parts, in which FIGURE l isa partial block diagram of a typical color i television receiver whereinthe average kinescope current j ab all is maintained constant and thepeak whites of the video signal are held just below blooming level;

FIGURE 2 is a schematic diagram illustra-ting the details of the twofeedback loops of FIGURE 1; and

FIGURE 3 is a partial schematic diagram of an alternative circuit thatmay be use in place of that employed in FIGURE 2 to maintain constantaverage kinescope current.V

FIGURE 1 shows, by way of illustration only, a typical televisionreceiver which may for example be similar to that described in PracticalColor Television for the Service Industry, published by RCA ServiceCompany Incorporated, Camden, New Jersey, second edition, April 1954. Inthe alternative, the receiver may be the same as that of the CTC7chassis, the circuits and service data for which are available from RCAService Company, Camden 8, New Jersey. While the specific form of thesignal processing apparatus does not constitute a part of the invention,the showing of a suitable receiver is made to fully and clearly setforth the environment in which the invention may operate. The groundsymbol has been omitted in the several blocks for the sake of claritybut may be assumed at present where needed to complete a circuit.

In FIGURE 1, a transmitted color television signal, received by anantenna 10, is applied to the input terminals of a television. signalprocessing section 12 of thereceiver. This signal processing section l2may include the usual radio frequency, mixer, and intermediate frequencystages of a typical television receiver. In the alternative,

' the television receiver signal processing section 12 may be consideredas the input of a composite color television signal from a suitablestudio signal source. In this case, the remainder of the circuitry inFIGURE l would then be termed a color television 'moniton The inventionas will be described below has equal utility with either a receiver or amonitor, monochrome or color.

The output of the television signal processing section 12 is passed to avideo detector 14 which detects the intermediate frequency signal fromthe processing section 12 to provide a composite black and white orcolor television signal as the case may be. In the instance of a colortelevision receiver, as has been assumed, the composite color televisionsignal from the video detector 14 is passed to the video circuits 16which may include a luminance channel and a chrominance channel. In thechrominance channel, the chrominance signal portion of the compositesignal is demodulated and matriXed to form the several red, green andblue color difference signals which are then applied to the respectiveguns of an image reproducing device or color kinescope 13.

The video circuit 156 also ampliiies the luminance and synchronizingpulses of the composite color television signal. The luminance signal,thus processed, is then applied to the red, green and blue guns of thecolor kineseope 18. The synchronizing components are passed fromV thevideo circuits 16 through a synchronizing signal (sync) separator andautomatic gain control (AGC) circuit which operates both toautomatically control the gain of the television signal processingsection t2 in a well known manner and also drive the vertical deiiectioncircuits 22. In addition, the synchronizing components pass through thesync separator 20 and drive the horizontal deflection and outputcircuits 24. As is well known, the sync separator 2@ also provides akeying pulse to the chrominance portion of the video circ-uit 16 toenable the color synchronizing burst to be separated from the compositecolor television signal in order that it might be used to control thedemodulation of the chrominance signal and thus derive the red, greenand blue color difference signals.

The color kinescope 18 includes a deection yoke 2d havingrterminals VVfor the vertical and HH for the horizontal windings of the yoke. Thevertical deflection windings VV are coupled to the output terminals WVof the vertical deflection circuits 22.

The horizontal deection and output circuits 24 drive a horizontal outputtransformer 28 of the high power voltage supply 39. The terminals HH ofthe horizontal deflection windings derived line frequency scanning wavesfrom output terminals HH of the horizontal output transformer 2S whichis energized by a current supplied by the horizontal output tube in thehorizontal output circuit 24. The horizontal output transformer 2S is ofthe auto-transformer type, the output of the horizontal output circuitbeing applied across a selected portion of the total series of windingsand the horizontal deection winding HH being effectively coupled acrossa small segment of this portion. A conventional damper tube 32 has itscathode connected to the high voltage transformer 2S and has its anodeconnected by way of an LC circuit 34 to a point of iixed referencepotential. In this case, the point of xed reference potential is that ofthe plate or B+ supply circuit 36. The lower portion ot the high voltagetransformer 28 includes a conventional B-Boost circuit 33 Y ployed asthe picture control voltage for adjusting theV level of the constantaverage kinescope beam current as will be described hereinafter. In thiscircuit the B-Boost voltage is a measure of the kinescope beam currentflowing through the kinescope ultor electrode 44.

The high voltage for the ultor electrode 44 of the kinescope 1.3 isprovided by high voltage rectier 46 the anode of which is connected tothe high potential terminal of the output transformer Sil. The ultorsupply electrode t4 is connected to the cathode of the high voltagerectifier 46..

Thus far described the circuit is that of a conventional televisionreceiver. In accordance with the invention, a small amount of additionalcircuitry is added to the conventional receiver which reduces spotblooming and power supply overload. Basically, the invention includestwo feedback loops, the rst being a loop which maintains the averagekinescope current constant. The second is a loop which acts to preventthe white peaks of the video from exceeding the level which causes spotblooming in the kinescope.

The latter of these loops, which may be termed the antiblooming loop,includes a simple high impedance diode circuit to sense the point atwhich spot blooming begins. The signal from this detector is amplified,rectitied and then applied to the automatic gain control circuit of thereceiver in such a manner as to turn down the over-v all set gain so asto reduce the amount of spot blooming.. Proper choices of theanti-blooming circuit gain, time constant, and sensing point are madesuch that the net effect is to hold the brightest portion of a givenscenev just below spot blooming.

Speciically, the luminance signal from the output of the video circuit16 is passed through a bandwidth limiting circuit 5,0 in order toprevent the blooming circuit from setting on narrow noise pulsesextending into the white picture region. The white peaks of theluminance signal, thus bandwith limited, are then referred to a bloomingreference voltage 52 by a blooming detection and amplication circuit 5d.Only the peaks above this reference level are allowed to pass and beamplied. These amplied white peaks are peak detected and the resultingDC. voltage is used to control the video gain of the video circuits le.Specific circuitry for accomplishing this function is described indetail in FIGURE 2. With such system, the output video or luminancesignal has its absolute amplitude controlled so that the white peaks arey allowed to exceed the blooming reference level by approximately 1/(loop gain) of the normal (open loop) amount.

The automatic brightness control loop uses the boosted B voltage fromthe high Voltage power supply as an indication of the average kinescopecurrent. When a predetermined kinescope current is thus sensed, beyondwhich the power supply would be overloaded resulting in the rasterbecoming defocussed, the D.C. bias of the video output stage is changedin the proper direction to reduce the kinescope current. This controloperates in both directions in such manner that the bias on thekinescope (brightness) is altered to maintain constant average kinescopecurrent. Because the kinescope average current remains constant, theultor power supply sees a constant load thereby eliminating the need forthe conventional shunt regulator to be in the power supply.

Specifically, the kinescope current controlling loop receives from thehigh voltage power supply 30 an electrical signal proportional to theamount of average kinescope current flowing. This signal as mentionedpreviously is derived from the arm of the potentiometer 42 which forms aportion of the series voltage divider 40 connected to the B-Boostcircuit 38. If this electrical signal, which is proportional to theB-Boost voltage, varies from a predetermined setting thereby indicatinga change in the average kinescope current an ultor current controlcircuit 56 operates to vary the bias on the kinescope (which controlsbrightness) to maintain constant average kinescope current.

In one form of the invention, the ultor current control circuit 56 usesvertical retrace pulses from the vertical deflection circuits 52 whichare allowed to pass through a gate controlled by the B-Boost voltage inthe varying amounts depending upon the value of the B-Boost voltage.These vertical retrace pulses are then amplified and detected to obtainthe variable D C. feedback signal which is used to control the bias ofthe video circuits `16, which in turn control the bias of kinescope 18.In another form of this invention, the ultor control circuit 56 may usehorizontal retrace pulses.

From this description it may be demonstrated that the reproduced imagewould be somewhat dependent upon the picture content since a picturecontaining, for eX- ample, low peak whites causes the video gain to beincreased due to the operation of the anti-blooming circuit. Because ofthe constant average kinescope current control, the brightness orbackground may be lowered to maintain the average kinescope current at aconstant value. The viewer will have the impression that he is watchingan A.C. coupled receiver. But such receiver will have both freedom fromoverload and also freedom from blooming and at a reduced cost because ofthe omission of the relatively expensive shunt regulator tube.

In FIGURE 2, the details of a conventional color television receivercircuit (here the RCA CTC7 referred to above), modified to accommodatethe teachings of the subject invention, is illustrated. In this figure,only the two control loops are illustrated. The constant current loopbegins with the circuitry in the upper left of the diagram. Varyingamounts of the vertical retrace pulses from the vertical deflectioncircuits 22 (FIG. l), are allowed to pass the diode 64 to be amplifiedby triode amplifier eil and then peak detected by a second diode 66. Thedetected signal from the diode d5 is filtered by the RC combination 68and then used to vary the bias on the control electrode of the secondvideo amplifier 62. Since the second video amplifier 62 is D.C. coupledto the cathode electrodes 7b of the kinescope i8, the bias on the gridof the second video amplifier varies the bias on kinescope liti whichdetermines the average ultor current drawn by the kinescope thuscompleting the constant current loop.

The portion of the vertical retrace pulses that are allowed to pass thediode 64 is determined by the value of the B-Boost voltage applied tothe terminal 72 which is derived from the B-Boost circuit 38 (FIG. 1).The variable potentiometer 42, which corresponds to that illustrated inFIG. l, provides a picture control whereby the portion of the B-Boostvoltage used to bias the diode 64 is varied. In this manner, dependingupon the bias applied to the anode `of the diode 64, varying amounts ofthe positive going vertical retrace pulses are allowed to pass to thetriode amplifier 60.

In operation, as the B-Boost voltage drops from a more positive value toa less positive value thereby indicating an increased average kinescopecurrent, the positive bias applied to the anode of the diode 64decreases thereby allowing a larger portion of the vertical retracepulses to pass to the triode amplifier 60. These increased amplitudepulses, after amplification, detection, and filtering increases thenegative bias applied to thecontrol grid of the second video amplifier62. This in turn increases the plate voltage of the second videoamplifier 62 thereby decreasing the average kinescope ultor current tomaintain the average kinescope constant. A similar description of theoperation can be given for the situation where the B- Boost voltageincreases indicating a decrease in average kinescope current.

The anti-blooming circuit receives its input from the plate of thesecond video amplifier 62 in the form of a sync positive signal asindicated by the waveform 74. Simply stated, the anti-blooming circuitseparates the white peaks 76, which would cause spot blooming, from therest of the video signal 74 by means of two diodes '78 and 0. Theseparated white peaks 76 are amplified by triode S2 and peak detected bya diode 84 to provide a D.C. control signal. This control signal iscoupled to the first video amplifier $6 and causes the AGC voltagederived from the plate of video amplifier Sti, to decrease the overallset gain, thereby reducing the spot blooming.

The blooming sensing circuit just described has several unique features.To begin with, there must be a blooming reference voltage to which thepeak white signals may be compared in orderto ascertain the amount bywhich the video white peaks exceed the blooming level. Spot blooming maybe determined, to a good approximation,`

with respect to the kinescope red guns grid to cathode voltage. This redgun voltage is in the minus 25 to minus 50 volt range in a typical colortelevision set of the CTC7 variety. Since the kinescope cathodes in thetypical color television sets are about 250 volts above ground, if thered background voltage is altered by only 10% (about 25 volts) theblooming reference voltage, if a constant voltage is used, is altered by50 to 100% as compared to the minus 25 to minus 50 volt grid to cathodeblooming voltage. It is thus apparent that to have a desirable bloomingcontrol circuit operation the blooming reference voltage must track thered background voltage which may be changed in order to compensate forthe vtube aging, etc.

Thus, in accordance With this invention, the blooming reference voltageVR is obtained from the red background potentiometer 88. Thus, changesin lthe setting of the red background potentiometer 88 are refiected inthe blooming voltage VR maintaining a constant blooming reference withrespect to the red grid to cathode voltage. Stated in another manner,the blooming reference level is made to track the red backgroundcontrol. It may -be noted in passing that prior to passing to theblooming detector, including the diodes 78 and Sil, the video signal 74is first passed through a low pass RC filter 90. This low pass filterlimits the signal fed to the blooming detector to about 500 kilocyclesand effectively reduces the effect of impulse noise.

The anti-blooming circuit illustrated in FIG. 2 also includes a novelmethod of altering the direct current appearing between the grid andcathode of the boot strapped first video amplifier 86. As will berecalled from the above description, since the gain of the LF. stages ofa television receiver is already controlled by an automatic gain control(AGC) system, in order to prevent spot V'capacitor C4.

blooming, a very low frequency (hence forth called D.`C.) Qontrol signalis applied to the boot strapped first video amplifier.' This first videoamplifier provides DC. gain to the control signal which alters Vthe AGCvoltage such as to increase or decrease the IF. gain of the televisionreceiver in the appropriate direction. However, inserting a DC. controlsignal into a boot strapped'video amplifier is an extremely difficultproblem.

More specifically, the problem is to insert the D.C. control voltage,which is referred to grid-to-cathode of first video amplifier, without:(l) having the boot strappo amplifier cathode resistor attenuate theD.C. gain of the stage as in a cathode follower, (2) havingy the videooutput signal peak detected in the D.C. control circuit thereby causingerroneous DC. control, (3) A.-C. shorting the boot-strapped cathoderesistor, (4) A.-C. grounding the IF. thus reducing the video gain tounity, (5) the DC. from the boot strapped video biasing the diode in thecontrol circuit.

The circuit illustrated in FIG. 2 overcomes these difficulties. Each ofthese difficulties are now considered in succession by describing themanner in which they are overcome by the illustrated circuit. Thecathode resistor 94 of the boot strapped first video amplifier 85 doesnot effectively reduce its D.C. gain because the majority of the D.C.control from the diode 84 appears across the resistor R2 which iscoupled between the cathode and control electrode of the video amplifier36. The second difficulty referred to above is alleviated since thevideo output signal cannot appear across the diode 84. The resistors R3and R4 and capacitors C3 and C4 from a balanced network so that bothterminals of the diode 84 are balanced with the same proportion of thevideo output signal. In this connection the capacitor C2 bypasses theresistor R2 for video frequencies to allow equal proportions of thevideo output signal to be applied equally to both terminals of diode 84.The third difficulty mentioned above is alleviated since the resistor R4is much greater than the cathode resistor 94. This means that thecathode resistor 94 is not A.-C. Shorted to ground by the The fourthdifficulty mentioned above is alleviated since the resistor R3 is muchgreater than the cathode resistor 94 and also since the capacitor C2bypasses the resistor R2 for video frequencies thus preventing the videofrom the LF. amplifiers from being A.C. grounded by the capacitor C3.The fifth difhculty mentioned above is alleviated since the capacitorsC3 and C4 prevent any D.C. biasing of the diode 84 by the boot strappedvideo stage.

In FIG. 3 there is illustrated an alternative arrangement which may beused in order to maintain constant average kinescope current in lieu ofthat circuit portion of FIG. 2 which is enclosed in the dottedrectangle. In accordance with the circuit of FIG. 3 a sample of thekinescope current is obtained and the voltage this current producesacross resistor 99, is subtracted from a reference voltage Mtl. Afiltered voltage proportional to the difference is then applied to thecontrol grid of the second video amplifier 62 (FIG. 2) and fed to thecathode electrodes "itl (FIG. 2) of the guns of the color kinescope I3.In this manner, the average kinescope current is maintained at aconstant value.

In FIG. 3, the horizontal output tube from the block 24 (FIG. l) drivesa high voltage and deflection transformer 96 which has an isolated highvoltage winding 98. In this manner, the kinescope ultor current iscaused to fiow through a potentiometer 99 to a voltage reference pointi). The potentiometer 99 is variable and constitutes a picture controlpotentiometer. The voltage drop occurring across the potentiometer 99subtracts from the voltage appearing at the voltage reference pointlili? which, for'example, may be B+, and the difference is applied tothe grid of the second video amplifier e2 (FIG. 2).

As in the case of the circuit operation of FIG. 2 this difference outputsignal is applied to the cathode "itt (FIG. 2) of the color kinescope 18(FIG. 2) thereby Varying the kinescope current. This complete.v circuitforms a highly degenerative feedback loop which Will reduce thedeviation of the average kinescope current, due to a changing videosignal, by l/ loop gain. Eecause the gain of the second video amplifier,the loop gain is sufiiciently high so that the average kinescope currentmay be considered a constant.

By way of example, to understand the circuit operation let it be assumedthat the average kinescope current increases. This results in increasedcurrent flow through the potentiometer 99. The Ivoltage from the grid tocathode of the second video amplifier 62 (FIG. 2) drops. With reducedvoltage at its control grid, the plate of the second amplifier 62 (FIG.2) increases in a positive direction thereby decreasing the kinescopecurrent. By this technique the assumed increase in average kinescopecurrent is nullified by the subsequent increase resulting from thefeedback, thereby maintaining an essentially constant average kinescopecurrent.

A novel and improved television system has been described which includesnot only a feature for maintaining the average kinescope constant butalso includes antiblooming circuitry for preventing spot blooming fromoccurring due to peak whites in the video signal. This invention resultsin a relatively low cost circuit which eliminates the need forregulating the power supply voltage and also provides a circuit which isquite easy for the inexperienced viewer to operate under optimum viewingconditions. The circuits of this invention find utility in eithermonochrome or color television receivers and monitors.

What is claimed is:

1. In a cathode ray tube system including a cathode ray tube devicehaving a beam intensity controlling means and an ultor electrode, a highvoltage supply coupled to said ultor electrode and means for'applying avideo signal to said beam intensity controlling means, an automaticcontrol system comprising means coupled to said video signal applyingmeans for detecting picture information peaks in one direction of saidvideo signal to produce a control signal, and means coupled to saidvideo signal applying means and responsive to said control signal forcontrolling the amplitude of said video signal applied to said beamintensity controlling electrode as a function of the amplitude of saidpicture information peaks in said video signal whereby toprevent saidcathode ray tube from spot blooming.

2. In a cathode ray tube system including a cathode ray tube devicehaving a beam intensity controlling electrode, and means for applying avideo signal to said beam intensity controlling means, an automaticcontrast control system comprising means coupled to said video signalapplying means for detecting peaks in one direction of said videosignal, said detecting means including a low pass filter means forpreventing noise peaks in said videol signal from being detected, andmeans coupled to said video signal applying means for controlling theamplitude of said video signal applied to said beam intensitycontrollingmeans as a function of the amplitude of said peaks in said video signalwhereby to prevent said cathode ray tube from spot blooming.

3. In a cathode ray tube system including a cathode ray tube devicehaving a beam intensity controlling elecrode and an ultor electrode, ahigh voltage supply coupled to said ultor electrode, and means forapplying a video signal to said beam intensity controlling electrode, anautomatic brightness and contrast control system comprising meanscoupled to said video signal applying means for detecting peaks in onedirection of said video signal with respect to a reference voltagelevel, means for deriving said reference level from said beam intensitycontrolling electrode whereby said reference level is caused to trackthe operating point of said beam intensity controlling electrode, andmeans coupled to said video signal applying means for controlling theamplitude of said video signal applied to said beam intensitycontrolling electrode as a function of the amplitude of said peaks insaid video signal whereby to prevent said cathode ray tube from spotblooming.

4. In a television system including a kinescope having a beam intensitycontrolling electrode and an ultor electrode, a high voltage supplycoupled to said ultor electrode and means for applying a video signal tosaid beam intensity controlling electrode, an automatic brightness andcontrast control system comprising means for sensing the average currentpassing to said ultor electrode from said high voltage supply, meanscoupled between said sensing means and said beam intensity controllingelectrode for controlling the direct current level of said beamintensity controlling electrode inversely as a function of the currentpassing to said ultor electrode whereby to maintain the average ultorcurrent constant, a source of a blooming reference voltage, meanscoupled to said video signal applying means and to said source ofblooming reference voltage for detecting low frequency white peaks ofsaid video signal to provide a blooming error signal, and automatic gaincontrol means coupled to said video signal applying means and responsiveto said blooming error signal for controlling the gain of said videosignal applying means inversely as a function of said White peakswhereby to prevent said cathode ray tube from spot blooming and wherebysaid kinescope may be allowed to operate under conditions of maximumbrightness and contrast.

5. In a color television signal receiving system, the

l combination comprising, an image reproducing device,

signal translating means for processing a received signal to provide avideo signal, means coupled to said Video signal amplier to detect whitepeaks appearing in said video signal, and automatic gain control meanscoupled between said detecting means and said signal translating meansfor controlling the gain of said signal translating means whereby toprevent the amplitude of said white l@ peaks from exceeding theamplitude level that causes spot blooming in said image reproducingdevice.

6. In a coior television signal receiving system, the combinationcomprising, a color kinescope having guns for generating at least oneelectron beam, signal translating means for processing a received signalto provide a video signal, a source of blooming reference potentialproportional to the bias on one of said guns at which spot bloomingoccurs in said kinescope, gating means for comparing the white peaks ofsaid video signal to said blooming reference potential for selectivelypassing only that portion of said white peaks that exceeds said bloomingreference potential, means coupled to said gating means for peakdetecting said white peaks that pass said gating means thereby toprovide a D C. control potential, and automatic gain control means beingresponsive to said DC. control potential being coupled to said signaltranslating means for controlling the gain of said signal translatingmeans for controlling the gain of said signal translating meansinversely with respect to said white peaks thereby to prevent theamplitude of said white peaks from exceeding that amplitude that causesspot blooming in said kinescope and whereby said blooming referencepotential tracks the operating characteristics of one of said guns.

7. The combination set forth in claim 6 which includes a low pass filtercoupled between said gating means and said video amplier thereby toprevent said automatic gain control means from operating in response tohigh frequency noise components that appear on said video signal.

References Cited by the Examiner UNITED STATES PATENTS 2,414,228 1/47Gottier 178-7 XR 2,892,028 6/59 Pritchard et al. l78-7.3

DAVID G. REDNBAUGH, Primary Examiner.

1. IN A CATHODE RAY TUBE SYSTEM INCLUDING A CATHODE RAY TUBE DEVICEHAVING A BEAM INTENSITY CONTROLLING MEANS AND AN ULTOR ELECTRODE, A HIGHVOLTAGE SUPPLY COUPLED TO SAID ULTOR ELECTRODE AND MEANS FOR APPLYING AVIDEO SIGNAL TO SAID BEAM INTENSITY CONTROLLING MEANS, AND AUTOMATICCONTROL SYSTEM COMPRISING MEANS COUPLED TO SAID VIDEO SIGNAL APPLYINGMEANS FOR DETECTING PICTURE INFORMATION PEAKS IN ONE DIRECTION OF SAIDVIDEO SIGNAL TO PRODUCE A CONTROL SIGNAL, AND MEANS COUPLED TO SAIDVIDEO SIGNAL APPLYING MEANS AND RESPONSIVE TO SAID CONTROL SIGNAL FORCONTROLLING THE AMPLITUDE OF SAID VIDEO SIGNAL APPLIED TO SAID BEAMINTENSITY CONTROLLING ELECTRODE AS A FUNCTION OF THE AMPLITUDE OF SAIDPICTURE INFORMATION PEAKS IN SAID VIDEO SIGNAL WHEREBY TO PREVERNT SAIDCATHODE RAY TUBE FROM SPOT BLOOMING.